The invention relates to a rapid-action agent for preparing cold and hot beverages from drinking water, in particular coffee or tea. Taking into consideration the disadvantages of known prior art, the aim of the invention is to provide a rapid-action agent for preparing cold and hot beverages from drinking water, said agent being easy to use, having a uniform, consistent action, eliminating the risk of microbial formation and noticeably improving the taste of the beverage after a relatively short treatment time. To achieve this, the inventive agent consists of cellulose fibres, which are modified by a chemical reaction, forming phosphate ester groups and which have an ion-exchange capacity of at least 50 mg copper per g dried fibres, said agent being brought into contact, at least temporarily for a period of several minutes, with the drinking or infusion water by immersion. The cellulose fibres are chemically modified by phosphorylation using phosphoric acid or ammonium phosphate, until a phosphor content of between 3 and 8 mass % has been obtained. To increase the mechanical stability of the fibres, the phosphorylation can also be combined with a urea treatment, until a nitrogen content in the form of carbamide groups of at least 1 mass % is obtained. The modified cellulose fibres have such a rapid-action that after immersion in the water of the preparation, a noticeable improvement in the taste of the drinking water and the beverages prepared from said water is achieved after only a few minutes, optionally aided by a light stirring of the liquid, without adversely affecting the taste of the respective beverage.